Apparatus for thermal swing adsorption and thermally-enhanced pressure swing adsorption

ABSTRACT

The present invention provides compact adsorption systems that are capable of rapid temperature swings and rapid cycling. Novel methods of thermal swing adsorption and thermally-enhanced pressure swing adsorption are also described. In some aspects of the invention, a gas is passed through the adsorbent thus allowing heat exchangers to be very close to all portions of the adsorbent and utilize less space. In another aspect, the adsorption media is selectively heated, thus reducing energy costs. Methods and systems for gas adsorption/desorption having improved energy efficiency with capability of short cycle times are also described. Advantages of the invention include the ability to use (typically) 30-100 times less adsorbent compared to conventional systems.

RELATED APPLICATIONS

This application is a divisional of Ser. No. 09/845,776, filed Apr. 30,2001, now U.S. Pat. No. 6,630,012, which is incorporated herein byreference.

This invention was made with Government support under contractDE-AC0676RLO 1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to apparatus and methods of gas adsorption.

BACKGROUND OF THE INVENTION

Separations of gases have long been important in gas purificationprocesses such as used industrially in gas purification. Removal ofcarbon dioxide continues to be an important objective for purifying airfor humans to live underwater and in space. Other important technologiesthat can utilize improvements for gas separation include: fuel cells,ammonia production, fertilizer manufacture, oil refining, syntheticfuels production, natural gas sweetening, oil recovery and steelwelding.

The adsorption capacity of a gaseous species onto an adsorbent iscommonly expressed in graphical form in adsorption isotherms andisobars, which are widely published in the literature and by adsorbentmanufacturers and suppliers. For the sorption of gas species, thecapacity is typically expressed as the equilibrium mass of the speciessorbed per unit mass of adsorbent (e.g., kg species/100-kg adsorbent).The sorbent capacity varies as a function of temperature and the partialpressure (concentration) of the species being sorbed. Loading orcapacity typically increases as the adsorbent bed temperature decreasesor the partial pressure of the sorbed species in the gas phaseincreases.

The variation of adsorption capacity with temperature and pressure canbe used to effect separations of gas species. For example, in pressureswing adsorption (PSA) gas species are adsorbed onto a sorbent atrelatively high pressure, tending to remove the species from the feedstream. In a regenerative PSA process, reducing the absolute pressure(e.g., applying a vacuum) to the loaded sorbent bed or reducing thepartial pressure of the sorbed species in the gas phase by sweeping alower concentration purge gas through the bed regenerates the sorbent.Cycle times for PSA processes are typically measured in minutes(Humphrey and Keller, “Separation Process Technology, McGraw-Hill,1997). In a regenerative temperature swing or thermal swing (TSA)adsorption process, species are adsorbed at low temperature where theloading capacity is relatively high and (at least partially) desorbed athigher temperature, thus recovering sorption capacity for additionalcycles.

In addition to gas species separations, TSA can be used tothermochemically compress gases. Sorption based thermochemicalcompression is applicable to refrigeration and heat pump cycles (e.g.,see Sywulka, U.S. Pat. No. 5,419,156) and for chemical processing ingeneral.

Gas adsorption is known to be applicable to a wide range of gas species(see, e.g., Kohl and Nielsen, Gas Purification, 5th Ed., Gulf Publ. Co.,Houston, Tex.). Kohl and Nielsen report that in conventional TSA gaspurification processes, adsorbent bed loading and unloading cycles aretypically on the order of hours.

Despite their long-known use and importance, multiple problems remainwith gas adsorption separation technologies. These problems include: useof excess energy, bulky apparatus or low capacity, cost, and slow rateand/or low mass of gas separated.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a sorption pump thatincludes an adsorption layer comprising an adsorption mesochannelcontaining adsorption media, and a heat exchanger in thermal contactwith the adsorption layer. The heat exchanger includes at least onemicrochannel. The adsorption layer has a gas inlet such that gasdirectly contacts the adsorption media without first passing through acontactor material.

In another aspect, the invention provides gas adsorption and desorptionapparatus that includes at least one adsorption layer comprising anadsorption mesochannel containing adsorption media. The adsorptionmesochannel has dimensions of length, width and height; wherein theheight is at least 1.2 mm. The apparatus possesses capability such that,if the adsorption media is replaced with an equal volume of 13× zeolite,having a bulk density of about 0.67 grams per cubic centimeter, and thensaturated with carbon dioxide at 760 mm Hg and 5° C. and then heated tono more than 90° C., at 760 mm Hg, then at least 0.015 g CO₂ per mL ofapparatus is desorbed within 1 minute of the onset of heating. By heatedto “no more than 90° C.” typically means that 90° C. water is passedthrough the heat exchanger; however, the phrase also encompasses heatingby other means such as an electrically-resistive heater. Preferably, theapparatus includes at least one heat exchanger in thermal contact withthe adsorption layer.

In yet another aspect, the apparatus is configured to selectively heatthe adsorbent. The at least one heat exchanger could be configured suchthat the heat exchange fluid flow paths substantially overlap the areaof adsorption channel or channels. Alternatively, the apparatus couldcontain a relatively thermally conductive material overlapping theadsorption channel or adsorption channels and a relatively thermallyinsulating material that does not substantially overlap the adsorbentchannel or adsorbent material. By “substantially overlap” it is meantthat, when viewed from a direction perpendicular to the direction offlow in which the adsorption channel and heat exchanger is stacked, theareas of the adsorbent channel(s) and the thermally conductive materialhave at least about an 80% overlap.

In a further aspect, the invention provides a sorption pump, thatincludes an adsorption layer comprising an adsorption channel containingadsorption media, and a mesochannel heat exchanger in thermal contactwith the adsorption layer. The mesochannel heat exchanger has a fluidflowing therethrough that has a high thermal diffusivity, such that thecharacteristic heat transport time for the fluid in combination with themesochannel heat exchanger is no greater than 10 seconds.

The invention also provides an apparatus in which adsorption/desorptioncells are connected to improve overall energy efficiency. Each cellcontains at least one adsorption mesochannel having an inlet and/oroutlet. Typically, each cell contains multiple adsorption mesochannelsthat share a common header and common footer, and that are operatedtogether. Preferably, each adsorption channel is in thermal contact withat least one heat exchanger. Each adsorption channel contains adsorptionmedia. Typically, the apparatus also contains or is used in conjunctionwith a heat source and a heat sink. In some embodiments, the heat sinkcould be the non-adsorbed gas, which is passed through and removed fromthe apparatus. The apparatus contains heat transfer conduits betweeneach cell and the heat source and heat sink and also contains heattransfer conduits between each cell and at least two other cells. Inoperation, the conduits carry a heat exchange fluid or can contain athermally conductive material. The apparatus also contains valves thatcan control gas flow into the at least one adsorption channel. Cellvolume is defined as the volume of the adsorption channel or channelsthat are operated together, including the volume of the heat exchangechannel or channels, the volume between such channels, the volume of theouter walls of the cells, and the volume of inlet and outlet footers,when present.

The invention further provides a method of gas adsorption anddesorption, comprising passing a gas into an adsorption layer where atleast a portion of the gas is adsorbed onto adsorption media to form anadsorbed gas and selectively removing heat from the adsorption layerthrough a distance of 1 cm, preferably 2 mm, or less into a heatexchanger; and, subsequently, selectively heating the adsorption mediathrough a distance of 1 cm, preferably 2 mm, or less from a heatexchanger, and desorbing gas. The gas directly contacts the adsorptionmedia without first passing through a contactor material. For more rapidheat transfer (and thus faster cycling), the adsorption channel maycontain heat transfer agents such as metal fins or pins, or graphitefibers.

The invention also provides a method of gas adsorption and desorption,comprising passing a gas into an adsorption layer where at least aportion of the gas is adsorbed onto adsorption media to form an adsorbedgas and selectively removing heat from the adsorption layer through adistance of 1 cm or less into a heat exchanger; and, subsequently,selectively heating the adsorption media through a distance of 1 cm orless from a heat exchanger, and desorbing gas.

The invention also provides a multi-cell sorption pump, comprising: atleast six sorption cells; where each sorption cell comprises at leastone adsorption layer, and at least one heat exchanger layer. Thermalconnections connect each sorption cell to at least two other sorptioncells and to a heat source and to a heat sink, such that each sorptioncell can cycle thermally from adsorption to desorption and back toadsorption by sequentially receiving heat from said at least two othersorption cells prior to receiving heat from the heat source, and thensequentially giving up heat to at least two other sorption cells priorto giving up heat to the heat sink, such that thermal recuperation isprovided.

The invention also provides a method of gas adsorption and desorption,comprising a first step of passing a gas into a first adsorption layercontaining a first adsorption media where at least a portion of the gasis adsorbed onto the adsorption media to form an adsorbed gas andremoving heat from the adsorption layer through a distance of 1 cm orless into a first heat exchanger. Subsequently, in a second step, theadsorption media is heated through a distance of 1 cm or less from thefirst heat exchanger, and gas is desorbed. Simultaneous with the firststep, a heat exchange fluid flows through the heat exchanger andexchanges heat with the adsorbent. This heat exchange fluid flows into asecond heat exchanger that, in turn, exchanges heat with a secondadsorption layer containing a second adsorption media.

The invention also provides a method of gas adsorption and desorptionthat includes: a first step of transferring heat from a heat source intoat least two first cells and desorbing gas from each of the two firstcells, and transferring heat from at least two second cells to at leasttwo third cells; a second step of transferring heat from the at leasttwo second cells to a heat sink, and adsorbing gas into the at least twosecond cells, transferring heat from the at least two first cells to theat least two third cells; a third step of transferring heat from a heatsource into the at least two third cells, and desorbing gas from each ofthe at least two third cells, transferring heat from the at least twofirst cells to the at least two second cells; and a fourth step oftransferring heat from the at least two first cells to a heat sink, andadsorbing gas into the at least two first cells, transferring heat fromthe at least two third cells to the at least two second cells. In thismethod, each cell comprises at least one sorbent, and at least one heatexchanger.

The invention also provides a method of adsorption and desorption thatprovides the thermal enhancement of PSA adsorption, thereby obtaininggreater utilization of the adsorbent media than would be accomplished byPSA adsorption alone. This includes cooling of the adsorbent mediaduring adsorption at one partial pressure of the adsorbing specie(s), sothat a greater amount of adsorbing specie(s) can be adsorbed, and/orheating of the adsorbent media during desorption at a lower partialpressure of the desorbing specie(s), so that a greater amount ofdesorbing specie(s) can be desorbed. In general, the methods describedherein are applicable for thermal swing adsorption, thermally-enhancedpressure swing adsorption, and thermochemical compression.

In a report (“Microscale Adsorption for Energy and Chemical Systems”)appearing on the PNNL web site in May 2000, Viswanathan, Wegeng andDrost reported the results of calculations and experiments forinvestigations of microchannel adsorption with short cycle times. Fromthe reported estimate that 95% of CO₂ reaches the zeolite particles in30 seconds, based on semi-infinite diffusion, it is clear that thiscalculation involves zeolite adsorbent in a “flow-by” arrangement,rather than a “flow-through” arrangement. A “flow-by” arrangement is onein which adsorbent occupies less than the full cross-sectional area ofthe flow path and gas flow is primarily by an adsorbent, requiring thatcontact of the adsorbent media, with the specie(s) to be adsorbed, occurprimarily by mass diffusion into and through the adsorbent structure,while in a “flow-through” arrangement the adsorbent is substantiallyplaced within the flow path, so that the fluid flows directly “through”rather than adjacent to the adsorbent structure.

Various embodiments of the invention can provide numerous advantagesincluding one or more of the following: rapid cycling, rapid sorbentregeneration, reduced time and/or larger volumes of gas adsorbed as afunction of sorbent mass required, excellent device stability, low cost,direct sorption into the sorption media without requiring diffusionthrough a contactor, preferential heating/cooling of the sorption mediato a greater degree than other elements of the adsorber structure,configurations of sorption units with recuperative heat exchange therebyallowing energetically efficient temperature swing separations and/ormore energetically-efficient, thermochemical compression.

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of this specification.However, both the organization and method of operation, together withfurther advantages and objects thereof, may best be understood byreference to the following description taken in connection withaccompanying drawings wherein like reference characters refer to likeelements.

GLOSSARY

In the present invention, the term “microchannel” refers to a channelwith at least one dimension, of 1 mm or less, preferably in a directionperpendicular to net flow through the channel. The term “mesochannel”refers to a channel with at least one dimension, in a directionperpendicular to net flow through the channel, of 1 cm or less.

The “theoretical capacity” of an amount of adsorbent is determined bymaintaining the adsorbent at a first temperature, at a fixed partialpressure for the gas specie(s) to be adsorbed, for a sufficient periodof time so that essentially no more gas will be adsorbed, then shuttingoff the gas flow and heating to a second temperature to desorb gas, atthe same or another fixed partial pressure for the gas specie(s), untilessentially no more gas is desorbed, and measuring the amount of gasdesorbed; the amount of gas desorbed is defined to be the “theoreticalcapacity” of an adsorbent material for that set of process conditions.The actual “capacity utilized” within a working sorption pump ismeasured at the same pressure and temperature conditions, but for aselected, finite period of time, and therefore may be less than thetheoretical capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a schematically illustrates a simplified adsorption process.

FIG. 1 b schematically illustrates a simplified desorption process.

FIG. 2 is a top down view of an adsorption layer having a serpentineadsorption channel.

FIG. 3 schematically illustrates a system and process for performingintegrated adsorption/desorption cycles.

FIGS. 4 a-4 f schematically illustrates a system and process forperforming integrated adsorption/desorption cycles.

FIG. 5 schematically illustrates a system and process for regeneratingair for an astronaut or the like.

FIG. 6 is an experimentally obtained plot of adsorbent temperature vs.time for multiple cycles of an inventive apparatus. Selectedmeasurements of gas volumes desorbed are indicated by the open circles.

FIG. 7 is an experimentally obtained plot of adsorbent temperature vs.time for one cycle of an inventive apparatus at varying flow rates ofheat exchange fluid through the heat exchanger.

FIG. 8 a is an experimentally obtained plot of adsorbent temperature vs.time for heating under comparable conditions for apparatuses that are:all-plastic, all-metal, and metal-plastic composite.

FIG. 8 b is an experimentally obtained plot of gas volume fractiondesorbed vs. time for heating under comparable conditions forapparatuses that are: all-plastic, all-metal, and metal-plasticcomposite.

FIG. 9 shows mass diffusion graphs. Predicted variation in relative CO₂concentration as a function of time (upper) and number of characteristicmass transport times (lower) for semi-infinite diffusion at a distance Lof 0.8 mm in a porous adsorbent (ε=0.5, τ=3, and D=1.67×10−5 m²/s).

FIG. 10 is a graph of predicted productivity of sorption pumps accordingto the present invention when tested under specified benchmarkconditions.

THEORY AND DESIGN OF MESOCHANNEL SORPTION PUMPS

A sorption pump is defined as a device which captures a gas, orconstituents within a gas, onto the surface of an adsorbent media, andthen desorbs at least a portion of the captured gas, as the system isbrought to a different temperature and/or pressure. A sorption pumpmakes use of the change in equilibrium sorption capacity of a sorbentthat occurs when temperature and/or pressure conditions are changed.

A mesochannel sorption pump contains adsorbent material within amesochannel, which is in thermal contact with a heat exchanger,preferably a mesochannel heat exchanger, thereby providing rapid heattransfer between the adsorbent mesochannel and the heat exchangermesochannel.

For example, CO₂ can be adsorbed onto the interior and exterior surfacesof zeolite, then heated to desorb CO₂ at a higher temperature(temperature swing adsorption—TSA). Alternately, CO₂ could be releasedfrom the zeolite at a lower pressure (pressure swing adsorption) thanthe pressure (or partial pressure) at which it had been adsorbed.

In order to obtain a sorption pump design that has a high productivityper hardware volume, it is necessary to cycle an adsorbing mediarapidly. This is encouraged by fast heat and mass transport, of the typethat can be provided by microchannels and mesochannels.

Heat and mass transport within fluids in microchannels and mesochannelsare usually dominated by diffusion; that is, since fluid flow inmicrochannels is almost always in the laminar flow regime (i.e., notturbulent flow), heat and mass transport are primarily obtained throughdiffusion within the fluids.

A mesochannel is a channel that has a height that is less than 1 cm, awidth, and a length, with the width and length not being limited by anydegree other than whatever is practical. A microchannel is a mesochannelthat has a height that is less than 1 mm. For both, the optimum designtypically includes orienting the height of the channel in the directionfor which rapid heat and/or mass transport is desired.

Rapid cycling of a mesochannel sorption pump requires attention to thetransient thermal response within the heat exchange channels, theadsorption channels, and the walls that separates these two sets ofchannels, especially if highly effective thermal recuperation isdesired, as will be discussed later. Preferably, the mesochannelsorption pump is designed so that the heat transport distance across thewalls is sufficiently small so that it does not significantly influencethe cycling time or performance of the system.

The characteristic heat transport time (t_(ht)) in a heat exchangerchannel is related to the time that it takes for a substantial degree ofthermal diffusion to occur. More precisely, for laminar flow within amesochannel, where heat transport is dominated by diffusion, thecharacteristic heat transport time is defined to be a function of theheat transport distance and the thermal diffusivity of the heat exchangefluid, as follows:t _(ht) =h ²/∝where h is the height of the channel and ∝ is the thermal diffusivity ofthe fluid. For example, water (at 300 K, 1 bar, with a thermaldiffusivity ∝=1.46×10⁻⁸ m²/sec) flowing in a 250 micron high channelwill have a characteristic heat transport time of:t _(ht) =h ²/∝=[(250 microns)(10⁻⁶ m/micron)]²/(1.46×10⁻⁸ m²/sec)=4.28secondsDefining the characteristic heat transport time in this manner ismathematically equivalent to setting the dimensionless Fourier Number(F_(o)) equal to unity. For processes where F_(o) is equal to one, asubstantial amount of diffusion has occurred; however, the transientthermal response of the fluid is not yet complete. Since net diffusionslows as thermal equilibrium is approached, additional time steps, oft_(ht) may be needed to achieve the desired approximation of thermalequilibrium within the heat exchanger channel.

By comparison, air (at 300 K, 1 bar, with a thermal diffusivity of∝=2.20×10⁻⁵ m²/sec) and liquid sodium (at 473 K, 1 bar, with a thermaldiffusivity of ∝=4.78×10⁻⁵ m²/sec) have characteristic heat transporttimes, in 250 micron high channels, of 2.84 milliseconds and 1.31milliseconds, respectively.

It is also useful to note that the characteristic heat transport timescales with the square of the channel height. With channels that are tentimes taller, that is, at 2.5 mm, the characteristic heat transporttimes increase by a factor of one hundred. Likewise, at one centimeterchannel height, the characteristic heat transport times for water, air,and liquid sodium for the above conditions are 6840 seconds, 4.55seconds, and 2.09 seconds, respectively. From this, it is clear thatliquid water does not approach thermal equilibrium with the channelwalls as quickly as air or liquid sodium (or, in general, compared togases or liquid metals). Air and liquid sodium therefore exhibitcharacteristic heat transport times within a 1 cm mesochannel that aresimilar in magnitude to the characteristic heat transport time forliquid water within a 250 micron microchannel, due to theirsubstantially greater thermal diffusivities.

It is desirable to select the design of the heat exchange mesochannelsuch that the combination of the channel height and the thermalproperties of the fluid are well matched with the thermal properties ofthe overall system.

The design of mesochannel sorption pumps also requires attention to masstransport within and into the sorption channel. For example, in manyapplications the expectation will be that the sorption pumpsubstantially removes the solute from the process fluid. For example, itmight be desirable to remove CO₂ from a combustion gas stream, or toremove an acid gas (e.g., H₂S, CO₂, etc) from a process stream.

A significant design tradeoff must be made for this type of process. Onedesire is to maximize the use of the adsorbent media capacity, nearlycompletely loading it with each cycle, and the other desire is to removeas much as possible of the solute from the feedstream. The adsorptionmedia will initially load more rapidly where it is close to the flowinggas stream; i.e., where the mass transport distances are very short.Complete, or substantially complete, loading of the adsorption bedoccurs last for portions of the bed that are furthest from the flowinggas stream. For this reason, the distance from the flowing gas stream tothe furthest section of the adsorbent media, measured normal to thedirection of flow, is of interest.

The characteristic mass transport time (t_(mt)) in an adsorbent channelis related to the time that it takes for a substantial degree of massdiffusion to occur into and within the adsorbent channel. Like thecharacteristic heat transport time, the characteristic mass transporttime, for a laminar flow system, is defined to be a function of the masstransport distance and the effective mass diffusivity of the solute(s)within the overall fluid, as follows:

 t _(mt) =L ² /D _(e)

where L is the mass transport distance and D_(e) is the effective massdiffusivity of the diffusing specie(s) within the overall fluid. Thecharacteristic mass transport time is therefore an attribute of fluidproperties, channel dimensions, and the structure of the adsorbentmedia. For the calculation of the characteristic mass transport time,the effective mass diffusivity is defined to be a function of the fluidmass diffusivity and the tortuosity factor and porosity of the adsorbentmedia. ThereforeD _(e) =Dε/τwhere D is the mass diffusivity of the adsorbent specie(s) in the fluid,and ε and τ are, respectively, the porosity and tortuosity factor of theadsorbent material in the adsorbent channel.

In calculating the characteristic mass transport time, it is importantto consider geometry. In general, two types of sorption systems are ofinterest. One type, called a “flow-through” system, directly flows thegas to be processed through the sorption channel. The other type, calleda “flow-by” system, flows the gas to be processed past the sorptionchannel; for a “flow-by” system, a contactor may be used, as describedin Drost et al., U.S. Pat. No. 6,126,723, to separate the adsorbentmedia from the channel that is directly flowing the gas. In this case,sorption occurs when the gas diffuses through the contactor and into theadsorbent media. Alternately, another “flow-by” system involves havingthe adsorption media arranged within the same mesochannel as is used toflow the gas, so that there is a preferential flow path that is adjacentto, but not directly through the adsorption media. For example, theadsorption media might be coated on the walls of the channel, or on an“insert” that does not take up the entire channel height.

For a contactor-based, “flow-by” system, where the adsorbent channel isessentially filled by the adsorbent structure, the height of theadsorbent channel is also the mass transport length within the adsorbentchannel. For a case with a 1 mm high adsorbent channel, where the ratioof porosity to tortuosity factor (ε/τ) for the adsorbent is ⅙, and themass diffusivity of the fluid is 1.67×10⁻⁵ m²/sec (corresponding to CO₂within a N₂ stream, at 298 K and 1 atmosphere pressure), thecharacteristic mass transport time is calculated to be:t _(mt) =L ² /D _(e)=(1×10⁻³ m)²(6)/(1.67×10⁻⁵ m²/sec)=0.359 secondsLikewise, if the channel had been 1 cm high, the characteristic masstransport time would have been calculated to be 35.9 seconds.

The evaluation of the characteristic mass transport time can aid in theconsideration of various mesochannel sorption pump configurations;however, additional details must be considered when designing a sorptionpump. To evaluate transient response, and cycle time, attention mustalso be paid to the chemistry of adsorption (including capacities andkinetic rates), the precise geometry and dimensions of the adsorbentchannel and the adsorbent media therein, as well as the percent oftheoretical capacity that the system is intended to achieve.

More generally, the cycling rate for mesochannel sorption pumps will bea function of chemistry, mass transport (including the mass diffusivityof the solute within the overall gas stream and within the adsorbentchannel), and heat transport (including the thermal diffusivity of thegas and solid material within the adsorbent channel, and thecharacteristic heat transport time for the combination of the adsorbentchannel, any heat exchange channel(s), and the structural material thatconnects them).

As described above, transport phenomena within mesochannels generallyexhibit characteristic heat and mass transport times betweenmilliseconds and tens of seconds. Systems of microchannels andmesochannels, in combination with appropriately chosen heat transferfluids, can therefore be designed that exhibit transient heat and masstransport response rates on the order of tens of seconds seconds orseconds, or faster, and mesochannel sorption pumps should therefore beable to operate through the complete TSA cycle within a few minutes orin some cases, within tens of seconds or less.

During the adsorption portion of the cycle, the adsorbed gas undergoes aphase change, and heat (the heat of adsorption) is released. Unless thisheat is removed as it is generated, it will cause a temperature risewithin the adsorbent bed, thereby limiting the amount of gas could beadsorbed. Likewise, during desorption, the evolution of the gas consumesenergy; unless the adsorbent bed is heated (corresponding to the heat ofdesorption), it will grow colder, thereby limiting the amount of gasthat can be desorbed.

Thermochemical Compressors

Based upon the above discussion, sorption pumps can be operated asthermochemical compressors. For these cases, heat is supplied (from aheat source) during desorption, and heat is removed (to a heat sink)during adsorption. In general, temperature swing adsorption isthermodynamically classified as a heat engine cycle, with the workoutput being the rise in pressure (or partial pressure) of the adsorbedgas species, or the degree of separation of the adsorbed gas speciesfrom the feedstream. Therefore, the operation and efficiency of asorption pump is governed by the same thermodynamics as other thermalpower cycles.

In principal, the highest theoretical energy efficiency is achieved in aheat engine if all elements of the heat engine cycle operate in athermodynamically reversible manner. This cannot be accomplished in areal-world system, but efforts are made to attain highly efficientsystems. For example, reversible heat transfer would require that theheat transfer occur across a negligible temperature gradient, with nopressure drop due to fluid friction. Heat exchangers have been builtwith low pressure drop and low terminal temperature differences, therebyproviding very high heat transfer effectiveness. While these units donot accomplish reversible heat transfer, the degree of irreversibilityis nevertheless minimized in such devices.

In general, the thermal efficiency of a heat engine is expressed as theamount of work that is produced (W) divided by the amount of heat thatwas put into the system (Q_(H)), from a heat source. Heat engines mustalso give up an amount of heat (Q_(L)) to a heat sink. A perfect heatengine, acting as a reversible system, is known as a Carnot Cyclesystem, with the following thermal efficiency (η_(th)):

 η_(th) =W/Q _(H)=(Q _(H) −Q _(L))/Q _(H)=(T _(H) −T _(L))/T _(H)

where T_(H) is the temperature of the heat that is delivered to the heatengine, and T_(L) is the temperature of the heat that is removed, withboth being expressed in absolute temperature scales, such as degreesKelvin or degrees Rankine. Again, this is the best that cantheoretically be accomplished. So, for heat engines operating betweentemperatures of, say 400 K (127 C) and 300 K (27 C), the highesttheoretical efficiency that could be obtained would be:η_(th)=(400−300)/400=0.25or 25%, which would be called the Carnot Cycle efficiency for a heatengine operating between these two temperature boundaries.

In actual practice, it is difficult if not impossible to approach CarnotCycle efficiencies for a thermochemical compressor, and it is especiallydifficult in a sorption pump based upon adsorption. Whereas most heatengines operate with a circulating fluid as the working media, asorption pump has a solid adsorbent mass as the working media. With eachcycle, the adsorbent media and its structural housing must be thermallycycled; together, this can require significantly more heat than whatwould be required if only the heat of adsorption was to be provided, andlikelwise can require that a substantially greater amount of heat beremoved to the heat sink.

The requirement to thermally cycle a mesochannel sorption pump canresult in a substantial energy penalty, because of the relatively highthermal mass of the system, unless effective thermal recuperation isprovided, In general, mesochannel and microchannel technology makes useof an architecture where the structural volume is typically 40% or moreof the overall hardware volume.

Many heat engine cycles make use of recuperative heat exchange as a wayto improve thermal efficiciencies. For example, a Brayton Cycle gasturbine system receives an efficiency boost if the exhaust of theturbine is used to preheat compressed air, before it is heated by aninternal combustion process. Likewise, an absorption-based,thermochemical compressor, which uses a liquid chemical solvent as thesorbent material, receives an efficiency boost if a recuperative heatexchanger is used to preheat the “rich” solvent prior to desorption,using heat from from the “lean” solvent after desorption.

Thermal recuperation can also be accomplished with adsorption-basedsorption pumps. For example, the schematic in FIG. 3 illustrates onepotential concept for a multi-cell, mesochannel sorption pump. The cycleis similar to one described by Sywulka, U.S. Pat. No. 5,419,156.Conceptually, the cells move clockwise through the cycle, while a heattransfer fluid circulates counter-clockwise through heat transferchannels in each cell. The highest temperature occurs in the cell at thetop of the diagram where desorption is occurring. As the heat transferfluid leaves this stage at its hottest temperature, it consecutivelygives up heat to the cells on the left that are cycling toward thedesorption step. At the bottom, the coldest cell is adsorbing. As theheat transfer fluid moves up through the cells on the right, it coolsthe cells moving down toward the adsorption step. In this manner, themajority of the heat associated with the thermal mass is effectivelyrecuperated. Some heat must be provided at the desorber and removed atthe adsorber, to make the system operate as a heat engine doingcompression work. Note that, in actuality, the cells may not physicallyrotate. Rather, virtual rotation can be accomplished by transitioningthe inlet and outlet points as well as the points where heating andcooling occur.

The concept of Sywulka requires a substantial amount of valving. Fluidpumps and valves for a thermally-recuperative, mesochannel sorption pumpcan be provided either by embedding the valves within the sorption pumpstructure or by connecting external valves to conduits that areconnected to the structure.

Other options for thermal recuperation exist for multi-cell sorptionpumps, with the overall goal still being to make use of thermal energyfrom a cell that is cooling, to support adsorption, and to provide heatto another cell that requires heating, to support desorption. As isshown in FIGS. 4 a-4 f, the continuous fluid process loop of Sywulka isreplaced with thermal connections between each cell and the cells thatare its immediate neighbors. The thermal connection can be made usingheat exchange fluid loops or by using thermal switches, for example.Again, fluid pumps and valves can be provided either internally orexternally.

Energy efficient operation requires that the recuperative heat exchangebe highly effective. It is preferred that the heat exchange channels andthe adsorption channels cooperate in a way such that at least 60%, ormore preferably, 80%, or more preferably yet, that 90% of the thermalenergy associated with operating the system is recuperated.

Thermally-Enhanced PSA

A mesochannel sorption pump can also perform PSA adsorption and, inprinciple, can provide for improved productivity of a PSA adsorptioncycle through thermal enhancement. As noted previously, adsorptionsystems typically generate heat during adsorption and consume heatduring desorption, cooling the adsorption media during that portion ofthe cycle. For conventional PSA systems, the heat of adsorption remainswithin the adsorption media, and the heat of desorption is taken fromthe adsorption media, cooling it. The net effect for conventional PSAsystems is that the theoretical capacity of the adsorption media isreduced, compared to if heat had been removed during adsorption and/oradded during desorption.

Thermal enhancement is not usually attempted for conventional PSAsystems, because of the very long heat transport distances, andaccordingly, the very much longer cycle times that would be required. Asnoted previously, conventional PSA systems typically have cycle periodsof minutes, whereas conventional TSA systems typically have cycleperiods of hours.

Mesochannel sorption pumps, however, offer shorter cycle periods thatare of similar magnitude as those for PSA systems. Accordingly, amesochannel sorption pump can perform thermally-enhanced PSA adsorptionand/or desorption, thereby providing enhanced utilization for a givenamount of adsorbent. More specifically, a thermally-enhanced PSAsorption pump, incorporating mesochannels for heat exchange andadsorption/desorption, provides cooling of the adsorbing media duringadsorption of a gas specie(s) at one pressure (or partial pressure)and/or heating of the adsorption media during desorption at a lowerpressure (or partial pressure). While this operation will typicallyrequire thermal energy for operating the process, the size of theadsorption system and the amount of adsorption media are reduced.Alternately, whereas in some applications the conventional PSA systemmight require high pressure operation for adsorption, requiringelectrical or mechanical energy to support the operation of compressors,a thermally-enhanced PSA mesochannel sorption pump could be operated aspart of a process cycle with a lower inlet pressure required, thereforereducing compressor power costs. In particular, this could be valuablefor an operation where there is a high value associated with electricalor mechanical power compared to a lower value associated with relativelylow temperature heat, especially if waste heat is available from anotheroperation at a suitable quality.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 a illustrates a simplified schematic of an adsorption process 2.Feed gas is fed in through tube 4 and valve 6, through inlet 7 intoadsorbent layer 8. Simultaneously with passing a gas through layer 8, acoolant flows through valve 9 and then through heat exchanger 10 whichremoves heat from adsorbent layer 8. Cooling is necessary because moregas is adsorbed at low temperatures and because adsorption generatesheat. Gas that is not adsorbed in the adsorption layer passes outthrough outlet 12 and valve 14. At the end of the adsorption cycle, feedgas is shut off.

An apparatus in the desorption mode is schematically illustrated in FIG.1 b. Heat exchange media control valve 9 is switched to pass arelatively hot fluid through the heat exchanger 10. Heat is requiredbecause more gas is desorbed at high temperature and desorption causescooling of the adsorbent layer 8. Valve 14 can be switched to redirectflow and, if desired, the desorbed gas can be collected.

FIG. 2 is a top-down view of an adsorbent layer 20 having a serpentineadsorption mesochannel 22. During operation, the adsorption mesochannel22 contains adsorption media (not shown). Heat exchange fluid headers 24can transport fluids to multiple layers of heat exchanger channels.

Although the adsorption channel 22 can take a variety of shapes, aserpentine configuration may be desirable for some applications. Theheight of the adsorption channel (height is the direction out of thepage in FIG. 2 and is measured from the bottom of the channel to thetop) is 1 cm or less, more preferably between 0.1 and 10 mm, and stillmore preferably between 1 and 5 mm. Controlling the height is importantbecause it limits the time for heat and mass transport and enablesfaster cycling time. The length of the adsorption channel 22 is in thedirection of the net flow and usually determined based upon the pressuredrop that can be allowed, and other considerations such as theapplication for which the invention is to be used. There is no limit onchannel length; however, for most applications the length of theadsorption channel is 25 cm or less, and more preferably 10 cm or less,and still more preferably between 1 cm and 6 cm. The width of theadsorption channel 22 is also a function of the design of the specificembodiment, and is often based upon internal design considerations, suchas the need for the walls that define the width of the channel to serveas structural ribs during the fabrication of the invention. The width isgenerally perpendicular to the height and length and is measured at anycross-section of the flow channel, and is not limited but is preferably10 cm or less, more preferably 5 cm or less, or still more preferablybetween 5 mm and 3 cm.

The structural material for the adsorption layer may be metal but ispreferably a low thermal mass material such as plastic. It has beenfound that a plastic adsorption layer combined with a metal heatexchanger interface to the adsorption channel results in superiordesorption properties measured as the fraction of gas desorbed as afunction of time, thus enabling faster cycling times. Preferably thechannel is cut completely through the adsorption layer and theadsorption media preferably contacts heat exchangers on two sides. Athinner adsorption layer reduces device size, weight and thermal mass(allowing more rapid temperature swings).

Gas adsorption media (which are solids) are well-known in the art andcan be selected for selectivity to the desired gas. For carbon dioxideand water vapor, 13× zeolite pellets are one preferred example. In orderto maximize capacity it is desirable to maximize the quantity ofadsorption media in the adsorption channel; however, there is a tradeoff with limitations on mass transfer rates—a more completely filledadsorption channel decreases the gas flow rate at a given pressure.Therefore, it is preferred to use pellets or particles such that gas canflow and diffuse through interstices between particles. Other preferredadsorbent media forms include porous, flow-through foams, felts andhoneycombs. In preferred embodiments, the adsorbent channel is more than50% filled, more preferably at least 80% filled, with adsorption mediameasured as a percent of the total volume of the adsorption channelwhere both particles and the accompanying interstitial space is countedas “filled.” In other preferred embodiments, the adsorption media fillsat least 60%, more preferably 80%, and still more preferably at least90%, of the cross-section (measured perpendicular to gas flow) of atleast one portion of the adsorption channel—in this fashion essentiallyall of a gas passing through even a short adsorption channel willcontact the adsorption media. Advantages of passing gas directly throughthe adsorbent include better opportunities to provide heating andcooling to the sorption media, since heating/cooling streams can beplaced on both sides of the media, and desorption occurs into a smallervoid space, therefore providing greater compression (for thermochemicalcompressor applications). In addition to the sorption media, in somepreferred embodiments, the adsorption channel also contains heattransfer agents such as fins or pins that project from the channel wallsor interspersed thermally conductive materials such as graphite or metalfibers or flakes.

The inventive apparatus preferably contains at least one microchannelheat exchanger in thermal contact with the adsorption layer. The term“microchannel” refers to a channel having at least one dimension of 1 mmor less. Preferably, the microchannels have a maximum height of 1 mm anda width of 10 cm or less and any length (length is direction of fluidflow through the channel), more preferably a height of 100 to 500micrometers (μm). In some preferred configurations, each adsorptionlayer is sandwiched between heat exchangers. The heat exchanger layerpreferably has a thickness (in direction of heat transport to/fromadsorption layer(s)) of 200 to 2000 μm, including the heat exchangefluid channel and the wall between the heat exchange channel and theadsorbent layer. Preferably, the heat exchanger is completely or atleast partially composed of a highly thermal conductive material; lowthermal mass for the heat exchange structure is also desired. The highlythermal conductive material of the heat exchanger preferably hasapproximately the same shape as the adsorption channel (e.g., serpentineshaped heat transfer material over and/or under a serpentine-shapedadsorption channel) because this configuration operates to selectivelyheat the adsorption media with reduced heating of other components ofthe device such as other portions of the adsorption layer, and may alsoincrease the thermal cycling rate. In some preferred embodiments, thehighly thermal conductive material of a heat exchanger overlaps at least80%, more preferably at least 90%, of the adsorption channel(s).Conversely, the adsorption channel(s) preferably overlaps at least 80%,more preferably at least 90%, of the fluid flow portion of a heatexchanger. “Overlap” is determined by viewing the device in thedirection of stacking and gauging the superposition of one element onanother. While devices have been tested with heat exchangers that areshaped with serpentine configurations that conform to the shape of theadsorption channel, the inventors also envision heat exchange layershaving a shaped, thermally-conductive microchannel regions withsurrounding areas of nonthermally-conductive material.

In place of, or in addition to channels for fluids, heat sources mayinclude electrically resistive heaters, light-absorbing surfaces orradioisotopes. Other process technology such as an exothermic chemicalreactor or a nuclear powered reactor, are also possible heat sources.Non-sorbed components of the gas that contacts the adsorbing media mayremove also remove heat during adsorption, therefore serving as a heatsink or as a portion of a heat sink. Alternately, the heat source and/orthe heat sink might each be part of a heat pump system, which elevatesthe low temperature heat from adsorbing layers to a higher temperatureheat, for desorption layers. In some embodiments, the apparatus isconfigured with other process technologies which produce low-temperaturewaste heat, such that the waste heat from the other process technologiesis the heat source for the inventive apparatus, or the heat sink for theapparatus might be another process technology that requireslow-temperature heat at approximately the temperature at which heat isremoved during or prior to adsorption.

The heat transport fluid in the heat exchanger is preferably water, butany suitable heat transfer fluid may be employed. For example, liquidmetals, hydrocarbon- and silicone-based fluids, many gases (such as air,nitrogen, carbon dioxide or hydrogen) and phase change fluids (such asrefrigerants) are also suitable heat transfer fluids for variousembodiments of this invention. As shown in the Examples section, higherflow rates increase the rate of thermal change and thus decrease cycletime; however, excessively high flowrates can increase thermodynamicirreversibilities, and therefore can cause the system to be less energyefficient.

For good heat transfer characteristics, compactness and ease ofconstruction it is preferred that the adsorption layer or layers and theheat exchanger or heat exchangers have planar surfaces or complementary(lock-and-key) surfaces such that the components stack on top of eachother.

In another aspect of the invention, a collection of adsorption cells arearranged so that recuperative heat exchange is provided, through theincorporation of a heat exchange fluid. FIG. 3 depicts one schematic forthis approach, with twelve mesochannel adsorption cells (each having oneor more mesochannels with adsorbing media and one or more heatexchangers). While one portion of the system is adsorbing, anotherportion is desorbing, and the remaining cells are either being preheated(for desorption) or cooled (for adsorption), using the heat exchangefluid. Heat exchange fluid from the desorbing cell (or cells) passesinto another cell (or cells) where it preheats an adsorbent—at the sametime, heat exchange fluid from the heat exchanger of an adsorbing cell(or cells) passes into another cell (or cells) where it precools anadsorbent. The heat exchange fluid could be routed through mesochannelsthat are embedded within each adsorption/desorption cell. This approachis similar to the concept that is described in an U.S. Pat. No.5,419,156 (incorporated herein by reference), which describes an overallconcept for adsorption compressors in general but does not apply them toadsorption/desorption using mesochannels or microchannels.

Another approach is schematically illustrated in FIGS. 4 a-4 f. Thisscheme preferably makes use of cells in multiples of 3. The illustrateddevice (seen from top down in each illustration) contains 9 cells, whereeach cell is represented as a box. In FIG. 4 a, heat is transferred fromheat source 40 along the path indicated by arrows 42 into cells 44. Theheat source 40 could contain, for example, hot fluid passing through apipe, a light-absorbing surface, an electrical resistor, or a containerwith a radioisotope, or a thermal switch or other thermal conduitproviding heat flow from another process technology. The cells 44,operating at the hottest temperature of the cycle, desorb gas 46 thatexit the cells through valved outlets (not shown). Simultaneously, warmcells 48 transfer heat to adjacent cold cells 50.

In a subsequent step, FIG. 4 b, the outlets of cells 44 are closed andheat from cells 44 is transferred to warm adjacent cells 50.Simultaneously, gas 52 is being adsorbed in cell 48 while heat 54 istransmitted to a heat sink (not shown). The heat sink could be, forexample, a coolant fluid in a heat exchanger, another process technologyrequiring heat at about or less than the temperature of adsorption, orsimply the atmosphere.

In a subsequent step, FIG. 4 c, heat from the heat source is transmittedto cells 50 which then desorb adsorbed gas. Simultaneously, warm cells44 transfer heat to adjacent cold cells 48.

In a subsequent step, FIG. 4 d, the outlets of cells 50 are closed andheat from cells 50 is transferred to warm adjacent cells 48.Simultaneously, gas 52 is being adsorbed in cells 44 while heat 54 istransmitted to a heat sink (not shown). Subsequent steps are shown inFIGS. 4 e and 4 f.

The invention also includes methods of making sorption apparatuscomprising the joining together the adsorption layer(s) and the heatexchanger. For all-metal units, bonding is preferably by diffusionbonding. Plastic-containing units may be bonded by adhesives, bycompression fittings, diffusion, or other methods. The apparatus maylikewise contain or be assembled from ceramics, and sealed using variousmethods.

The inventive gas adsorption methods all involve sorption of a gas ontoadsorption media followed by desorption. In one aspect, a gas is passedinto a channel that contains adsorption media (preferably without firstpassing through a contactor) and, simultaneously heat is transported toor from a heat exchanger to any point in the adsorption media over a netdistance of less than about 1 cm. The net effect is that the gas isconcentrated or compressed, with the energy for the process beingthermal energy (heat). Since a heat source and a heat sink are required,the thermodynamic cycle is that of a heat engine. Further, because theheat and mass transport distances are short, along the smallestdimension of microchannels and/or mesochannels, the system operates witha fast cycling rate. The longest heat transport distance from any pointin the adsorption media is less than about 1 cm; more preferably lessthan about 8 mm, and still more preferably less than 5 mm. In somecases, the heat transport distance could be larger. To enhance heattransport, a porous conductor could be placed in the adsorption channel.In another aspect, heat is selectively added and removed from theadsorption layer with relatively low level of heat transfer to otherportions of the adsorption layer. In yet another aspect, heat istransferred between a heat exchanger composed of a high thermalconductivity material and an adsorption layer composed of a relativelylow thermal conductivity material.

In yet another embodiment, gas is desorbed from the adsorbent in theadsorption channel by pressure swings. Although heat exchangers are notnecessary for pressure swing adsorption, there could be heat exchangersto enhance rate and/or capacity.

The invention also includes methods of gas separation that include theinventive gas adsorption methods as steps in the process. Examplesinclude separation of CO₂ from exhaled air, removal of H₂S from naturalgas to “sweeten” the gas, removal of CO₂ and/or CO from a hydrogen richstream (such as from a reformer) for a fuel cell power plant, waterremoval from air (to dry it) and more complex separations, where thesorption device is but one part of the process, such as to purify argonor nitrogen such as for instrument use.

Alternatively or in addition to describing the invention in terms ofsize, composition, etc., the invention can be described in terms ofother measurable properties such as rapid cyclability and gasadsorption/desorption as a function of hardware volume.

The use of the thin adsorption channel enables faster heat transfer,which can be expressed as the heat transfer power density. In order toenable rapid cycling, heat is exchanged between the adsorbent channelsand the heat exchange channels at a rate of at least 0.5 watts per cubiccentimeter, more preferably over a rate of at least 1.0 watts per cubiccentimeter, and yet more preferably over a rate of at least 5.0 wattsper cubic centimeter, measured over a complete cooling and heating cyclewhere the volume is the adsorption cell volume, as previously defined.The upper limit of heat transfer in the present invention is limited asthe thickness of the adsorption layer approaches zero. In some preferredembodiments, the rate of heat transfer is between about 1.0 and 6.0W/cc.

In some preferred embodiments, the inventive apparatus possesses rapiddesorption capability such that, if the adsorption media is replacedwith an equal volume of 13× zeolite, with a bulk density of 0.67 gramsper cubic centimeter, and this zeolite is saturated with carbon dioxideat 760 mm Hg while at 5° C., and then warmed to 90° C. (by passing 90°C. water through the heat exchanger(s)) while maintaining the output at760 mm Hg, then at least 50% of the theoretical capacity of the zeoliteis desorbed within 1 minute. More preferably, at least 70%, and stillmore preferably at least 90%, of the adsorbed carbon dioxide is desorbedwithin 1 minute. The invention can also be characterized byproductivity. In an alternative test, under the same conditions asabove, at least 0.015 g CO₂, more preferably at least 0.025 g and insome embodiments 0.015 to about 0.04 g CO₂, per ml of apparatus isdesorbed within one minute. The rapid desorption capabilities of theinvention are generally insensitive to the type of adsorbent media; thepurpose of characterizing certain embodiments of the invention in thisfashion is to provide a measurable criterion that can be used tocharacterize the hardware design and thus characterize the rapiddesorption property of the invention.

In preferred embodiments the invention possesses rapid cyclability suchthat at least 70% (more preferably about 80% to about 95%) of thetheoretical capacity of the adsorbent media is utilized in at least twoconsecutive adsorption-desorption cycles, each cycle being accomplishedin a period of two minutes, as measured by a test in which a pure gas ispassed into the adsorption channel for 1 minute, for the sorptionportion of each cycle, at a flow rate such that the total amount of gasequals 1.5 times the theoretical capacity (the capacity of a sorbent atinfinite time) of the adsorption media, and 10° C. water issimultaneously passed through the heat exchanger(s), and then the gasflow is shut off and the adsorbent layer(s) are heated, during thedesorption portion of each cycle, by passing 90° C. water through theheat exchanger(s) for 1 minute. In this measurement, the pure gas shouldbe selected to match a typical target gas for the selected adsorbentmedia.

The inventive systems can also exhibit excellent stability and, inpreferred embodiments, productivity decreases by less than 10% evenafter 100 cycles.

In typical applications, multiple adsorption layers are interleaved withmultiple heat exchanger layers into single units. Preferably, anintegrated unit will sandwich each adsorption layer between two heatexchangers. More preferably a unit will contain at least 5 adsorptionlayers and 6 heat exchange layers. In some embodiments, larger volumesof gases can be separated with units containing at least 50 adsorptionlayers interleaved with 51 heat exchangers.

In a preferred embodiment, a mesochannel adsorption cell is designed sothat adsorbent media can be added or removed after bonding. Theadsorption channel(s) can be made longer or disposed further to one sidethan the heat exchange channels. In this design, the cell can be opened(such as by cutting or removing bolts) and sorbent media removed and/oradded without opening the heat exchange channels. The unit would then beresealed by welding, compression fitting, or other methods.

An air treatment system for an astronaut or like is illustrated in FIG.5. The water adsorber may contain silica gel, or zeolite, or othersuitable adsorbent; likewise, the CO₂ adsorber may contain zeolite orother preferred adsorbent. For each specie to be adsorbed (e.g., waterand CO₂) there are two sorption cells, one that is used at a given timefor treatment and one that is being regenerated by addition of heat. Ina preferred embodiment, the system would consist of multiple sorptioncells for each specie to provide improved thermal recuperation.Simultaneous regeneration results in a reduction in required adsorbentmass as compared to a non-regenerated system using the same adsorbentmedia. The mass and volume of adsorbent required decreases linearly asthe cycle time decreases. For a typical 4-hour mission, a systemregenerated in 4-minute cycles requires about 60 times less adsorbentthan a conventionally-constructed non-regenerated system.

While the invention has been described with particular attention tocarbon dioxide, it should be recognized that the inventive apparatus andmethods are equally applicable to other gases. For example, by properselection of conditions and adsorption media, the inventive apparatusand methods could be used to separate, or modify the partial pressuresof: refrigerants, H₂S, CO, H₂O, CO₂, H₂, hydrocarbon gases, and manyother organic and inorganic gases or vapor species, etc.

EXAMPLES

Relatively small adsorption separation and thermochemical compressionunits are possible utilizing mesochannel adsorption beds and/or heatexchangers, because of improved rates of heat and mass transfer in smallscales. These improvements result in TSA devices that can be cycled morerapidly, which in turn reduces the mass of adsorbent necessary toachieve a target separation.

Example 1

Adsorbent Mass Reduction for Rapid Thermal Cycling

As a practical test case, consider the adsorption and desorption of pureCO₂ from zeolite 13× at near atmospheric pressure. A vendor-suppliedzeolite 13× isobar at 760-mm Hg CO₂ pressure shows the equilibrium CO₂capacity q varies nearly linearly with temperature T, over the range of−50° C. to 100° C., per the relationship $\begin{matrix}{q = {\frac{24.9 - {0.115T}}{100} = {0.249 - {0.00115T}}}} & (1)\end{matrix}$Here, the units of q are kg CO₂/kg zeolite and T is in degrees Celsius(C). The theoretical working capacity per cycle q_(w) for adsorption ata low bed temperature T_(l) and desorption at a higher temperature T_(h)under the isobaric conditions is therefore expressed as $\begin{matrix}{q_{w} = {{0.00115\left( {T_{h} - T_{l}} \right)} = {{0.00115\quad\Delta\quad T} = \frac{m_{desCO2}}{m_{ads}}}}} & (2)\end{matrix}$The working capacity q_(w) is the maximum amount of gas desorbed atT_(h) when the gas is adsorbed to equilibrium loading at T_(l).Experimentally, the actual working capacity was determined from theknown amount of zeolite in the adsorbent bed and the volume of CO₂desorbed, where the desorbed volume is related to the mass of CO₂desorbed through the ideal gas law.

In the simple case described above, a single sorption unit is operatedin an adsorption stage and a desorption stage according to the schematicshown in FIG. 1. The combination of a gas adsorption phase and adesorption phase defines a single cycle of duration t_(cyc). The massproduction rate of gas stripped from the feed gas and desorbed as“product” gas, CO₂ in this case, is simply given by $\begin{matrix}{r_{gas} = {r_{CO2} = \frac{m_{desCO2}}{t_{cyc}}}} & (3)\end{matrix}$Substituting Equation (3) into (2) provides a relationship between cycleduration and the mass of adsorbent required to achieve a givenproduction rate. $\begin{matrix}{m_{ads} = {\frac{r_{CO2}t_{cyc}}{0.00115\quad\Delta\quad T} = \frac{r_{gas}t_{cyc}}{q_{w}}}} & (4)\end{matrix}$Note the linear relationship between the mass of adsorbent needed toachieve the production rate and the cycle time. Thus, the rapid cyclesachievable in a mesochannel adsorption device reduce the amount ofadsorbent needed to process a given amount of gas. The mass of adsorbentcan be reduced by more than 100 times with the mesochannel approach,because the working capacity is reused frequently.

Several mesochannel adsorbers with integrated microchannel heatexchangers were tested under isobaric (atmospheric pressure, typically˜750 mm Hg) conditions and operated in a two-phase cycle. In one testcase, a stainless steel device (described in detail in Example 4)containing 1.2 g of zeolite 13×(180-212 μm particle size) was operatedwith a minimum adsorption temperature T_(l) of 12° C. and a maximumdesorption temperature T_(h) of 77° C. (Water was flowed through theheat exchanger at 80 mL/min, and the hot and cold reservoirs were set to90 and 5° C., respectively.) Pure CO₂ was fed to the zeolite at the rateof ˜50 mL/min during a ˜60 s adsorption phase in which the adsorbent wascooled from T_(h) to T_(l). The desorption phase, including the time toheat from T_(l) to T_(h), was also ˜60 s. As defined above, the cycletime t_(cyc) was 2 min. About 46 mL of CO₂ (measured at roomtemperature, ˜22° C.) corresponding to 0.084 g CO₂ was desorbed. Thetheoretical working capacity for these conditions as determined fromEquation (2) is 0.090 g CO₂. Therefore, 93% of the theoretical workingcapacity was measured experimentally. The less than maximum workingcapacity for the device is thought to be due to partial water“poisoning” of the adsorbent. Water is strongly adsorbed on zeolite 13×,and the adsorption device was not heated sufficiently to remove allwater before the CO₂ experiment.

FIG. 6 shows bed temperatures for a series of 1, 2, 6, and 10 minuteadsorption/desorption cycles. The volume of gas measured at the end ofeach desorption cycle is indicated by the open circles. The desorbed CO₂volume consistently reached 46 ml for the 6 and 10 minute cycle timesand about 42 ml for the 2 minute cycles. Only about 22 ml CO₂ wasdesorbed in the 1 minute cycle; however, only about 25 ml of CO₂ wasdelivered to the bed during the adsorption swing. Tests with higher feedflow rates resulted in larger recovered gas volumes. The theoreticalworking capacities, based on measured temperature differentials, wereabout 52 ml for the 6 and 10 minute cycles and about 47 ml for the 2minute cycle. Therefore, better than 80% of the theoretical workingcapacity was achieved in each of these cycles. As can be seen in thefigure, the devices exhibited excellent cycle-to-cycle consistency.

Example 2

Adsorbent Mass Reduction for a Thermochemical Compression Scheme

Example 1 describes an isobaric process, but many adsorption processesare not isobaric. Consider, for example, an application in which thegoal is to achieve CO₂ compression thermochemically using a mesochanneladsorption system. Here, it is proposed to use a thermal swingadsorption process to capture (adsorb) CO₂ at low temperature and lowpressure (e.g., ˜6 mm Hg) and deliver (desorb) CO₂ at higher temperatureand pressure (e.g., 760 mm Hg) to a fuel-producing chemical reactor.Since adsorption, not desorption, is favored by higher partial pressuresof the adsorbed gas species, it is necessary to operate the thermalswing over a sufficient temperature range to have a net production ofgas in each thermochemical compression cycle.

For demonstration purposes, we consider adsorption at −50° C. and 6 mmHg of CO₂ and desorption at 100° C. and 760 mm Hg of CO₂, to providegreater than 125-times compression. The experimental results discussedin Example 1 shows that commercially available adsorption isobars andisotherms for CO₂ on zeolite 13× can be used to successfully scale theadsorption system. The difference in the low and high temperatureequilibrium CO₂ capacity values obtained from the literature is thetheoretical capacity (q_(w)) per sorption cycle for these operatingconditions $\begin{matrix}{q_{w} = {0.0485 = \frac{m_{desCO2}}{m_{ads}}}} & (5)\end{matrix}$It follows then, per the discussion in Example 1, that the mass ofzeolite needed to produce compressed CO₂ at a given rate is linearlyrelated to the cycle time $\begin{matrix}{m_{ads} = \frac{r_{CO2}t_{cyc}}{0.0485\quad\eta_{ads}}} & (6)\end{matrix}$Where r_(CO2) is the rate of CO₂ produced at higher pressure and t_(cyc)is cycle time. The efficiency factor η_(ads) is included in Equation (6)to account for extra adsorbent mass that may be necessary if the systemis not operated at 100% capacity. This might occur if the system was notoperated until equilibrium conditions were achieved (i.e., the maximumq_(w) is not attained in each cycle) or if a fraction of the bedcapacity for CO₂ was lost to other species (e.g., water). Based onexperimental results for isobaric conditions, efficiency factors greaterthan 0.9 are possible in properly configured and conditioned mesochanneladsorption devices (see Example 1).

Assuming temperature and pressure operating limits as given above and anidentical efficiency factor of 0.9 for both processes, the adsorbentmass requirements for the two approaches can be compared. Per Equation(6), only ˜1.3 kg of zeolite 13× is needed to produce compressed CO₂ atan intermediate rate (e.g., r_(CO2)≅20 kg/day) in a mesochannel devicewith a cycle time of 4 minutes. On the other hand, about 60 times thatmass of zeolite (˜80 kg) is needed for an adsorption thermochemicalcompression process cycled once in four hours, as is more typical ofconventional TSA processes.

Example 3

Rapid Thermal Swing Adsorption Cycles in Mesochannel DevicesDemonstrated

FIG. 7 shows the rapid thermal-swing capability for an all-metalmesochannel adsorber in a series of 1-minute heating and cooling phases.(In separate tests, it was determined that the heat-exchange surfacemeasured temperatures depicted in the figure are representative of thezeolite bed temperature to within 1 to 2° C.). As the heat-exchangefluid flow rate was increased from 20 mL/min to 80 mL/min, the maximumand minimum adsorber temperatures approached the hot (70° C.) and cold(5° C.) reservoir temperatures. A larger temperature differentialbetween adsorption and desorption cycles increases the zeolite workingcapacity, and therefore, a higher adsorbent working capacity is expectedas the water flow rate is increased. FIG. 7 also shows that the approachto the maximum (or minimum) temperatures is faster with increasingheat-exchange fluid flow rate. The heating curves were fit toexponential decay functions and the exponential time constants wereestimated. The time constants were approximately 6 s, 9 s, and 19 s forwater flow rates of 80, 40, and 20 mL/min, respectively. These datavalidate, from a heat transfer perspective, the potential for rapidthermal cycling in mesochannel adsorbers.

In Example 1 given above and the experiments from which FIGS. 6 and 7were generated, the adsorption device was fabricated of all stainlesssteel components. Other adsorption test devices were fabricated ofplastic components or a combination of plastic and metal components. Thepurpose of the plastic was to reduce the overall mass as well as thethermal mass. Here thermal mass implies the mass of structure which mustbe heated and cooled in adsorption and desorption cycles—ideally onlythe adsorbent would be heated and cooled, not the surrounding structure.Reductions in thermal mass result from the use of lower density andinsulating materials (e.g., plastics). To enhance the rate of indirectheat transfer from a fluid contained in a heat exchange channel to anadsorbent material contained in an adjacent layer, it is preferred thatthe interface be fabricated of a low-mass, thermally-conductingmaterial. This benefit can be achieved with metal-plastic compositedevices and all-plastic devices with a relatively conductive heattransfer interface (e.g., a thin copper sheet or a conductive polyimidesheet).

Representative experimental devices, described in more detail in Example4, were tested under isobaric conditions per Example 1. FIG. 8 comparesthe thermal and mass transfer performance of three different mesochanneladsorption devices during desorption cycles. In all cases, water wasdelivered to the adsorber heat exchangers from a 90° C. reservoir at 80mL/min. FIG. 8 a indicates a somewhat lower rate of temperature changein the all-plastic device than in the all-metal and metal-plasticcomposite devices. However, after ˜70 s the temperatures in theall-plastic device match those in the all-metal device, and at longertimes the temperatures in the all-plastic device exceed those in theall-metal device by a few degrees. This may be due to the lower thermalmass and lower heat loss associated with the plastic unit. (It shouldalso be noted that the temperatures shown for the stainless steel devicewere measured on the external surface whereas the temperatures for theother devices were measured with hypodermic thermocouples embedded inthe zeolite channel. It is possible that the internal temperature in theall-metal device is actually up to a few degrees higher than theexternal surface.) The temperature profile for the metal-plasticcomposite adsorber demonstrates superior qualities compared to each ofthe other units. It shows rapid heat transfer to the adsorber bed, therate of temperature change exceeding the all-metal device after ˜15 s.The metal-plastic composite also attained a slightly higher maximumtemperature than the all-metal device, suggesting relatively low heatloss as in the all-plastic device. The profiles of gas evolution fromthese three devices (FIG. 8 b) show similar trends. Gas desorption wasfastest in the metal-plastic device and slowest in the all-plastic unit.The results suggest that heat transfer rather than mass transfer is theprimary limitation in the regeneration process of these flow-throughmesochannel adsorbers.

In FIG. 8 b the gas volume fractions represent the absolute gas volumesnormalized by the total volume desorbed. In general we observed that theabsolute volume of CO₂ desorbed per cycle, the working capacity, washigher in all-metal adsorbers (up to 93% of theoretical) than in devicescontaining plastic (maximum of 62% of theoretical). We believe this isdue primarily to lower device conditioning temperatures used withplastic-bearing units (˜125° C.) compared to all-metal devices (˜195°C.). Water vapor, which is sorbed on zeolite 13× during assembly ofadsorbers exposed to atmospheric conditions, is difficult to strip fromthe adsorbent at low temperatures because of the strong affinity ofzeolite for water. Even at 195° C. some water is adhered to zeolite. Theworking capacity for an all-metal device conditioned in a 195° C. ovenwas ˜81% of theoretical. Additional conditioning of the zeolite-filledadsorber at 195° C., including treatment in a nitrogen purged vacuumoven, resulted in a working capacity increase to ˜93% of theoretical.These CO₂ recovery results indicate that precautions that must be takenagainst water poisoning for low temperature operations with zeolitesorbents. Of course, the adsorption devices developed here are notspecific to zeolite adsorbents (or CO₂ processing), and other adsorbentsthat are less sensitive to water may be used.

Example 4

Details of Experimental Mesochannel Adsorption Devices

Details of fabrication of three experimental mesochannel adsorptiondevice types, stainless steel, plastic, and metal-plastic composite, aredescribed here. Note that FIG. 1 is an oversimplified schematic of thedevice architecture. In practice, heat exchange channels were mounted onboth sides of the adsorbent bed, not just on one side as shown in thefigure. A production mesochannel adsorption cell would (at least for thecase of a flow-through adsorption channel) likely consist of a series ofadsorbent channels layered between heat exchange channels such that eachadsorbent channel is contacted by two heat transfer surfaces. A commonheader and common footer would connect the adsorbent channels in theadsorption cell and a separate path would connect the heat exchangechannels. The specific design of adsorbent and heat exchange channels isnot restricted to those described here. The method of assembly for aproduction unit might also be significantly altered. For example, an allstainless steel device would likely be fabricated with diffusion bondingprocesses (as is typically employed to make many other microchannel andmesochannel devices) instead of using conventional welding or adhesives(e.g., RTV silicone) to join the various layers.

Stainless Steel Test Device

FIG. 2 illustrates an adsorption layer prior to final assembly. Theassembled device included two heat exchange channel assemblies (notshown) sandwiching the serpentine adsorbent bed shim (also termed asheet or laminae). During assembly the serpentine channel was filledwith zeolite or other adsorbent material. The components of thisexperimental test device were temporarily bonded using RTV silicone tofacilitate disassembly and reuse with different adsorbent. The heatexchange assemblies consisted of blank stainless steel header shims towhich gas and heat exchange fluid inlet and outlet tubes were welded.The heat exchange channel was formed adjacent to the header plate with amicrochannel heat exchange shim originally designed for anothermicrochannel device.

Sample assembly of an all-stainless steel adsorber: (a) 0.020-in (0.51mm) thick stainless steel header plate for fluid fed to and retrievedfrom the adsorbent channel including a porous metal screen cover overthe ports to prevent loss of adsorbent from channel (c); (b) ˜0.010-in.(0.25 mm) height heat exchange fluid channel etched in 0.020-in. thickstainless steel shim stock with the etched surface facing the headerplate (a); (c) a stainless steel serpentine adsorbent shim, typically0.060-in. (1.5 mm) thick; (d) another heat exchange fluid channel (b)with the etched surface facing the header plate (e); and (e) 0.020-inthick stainless steel header plate for fluid fed to and retrieved fromthe heat exchange fluid channels. The sepertine adsorbent mesochannelcould be made by a technique such as milling and the microchannels inthe microchannel heat exchanger can be made by a technique such aselectrodischarge machining. Alternatively, either can be made byphotochemical machining or other suitable machining techniques.

All-Plastic Test Device

Like the stainless steel device, the all-plastic mesochannel adsorberincluded two heat exchange microchannel assemblies surrounding aserpentine mesochannel adsorbent bed shim. In the plastic unit however,both the adsorbent shim and heat exchange channels were fabricated ofpolyimide, and the header plates were made from a transparent plasticsuch as polycarbonate. Heat exchanger shims in the all-plastic and theplastic/metal composite devices were patterned using a ResoneticsMaestro UV excimer laser machining station operated at a wavelength of248 nm. The serpentine design of the heat exchange channel tracked theadsorbent channel to maximize effective heat transfer to the adsorbent.The various device layers were assembled with thin sheets ofdouble-sided adhesive film cut in the appropriate pattern. The unitswere pressed in a lab press to promote bonding.

An all-plastic adsorber was assembled with the shim order: (a) 0.25-inthick polycarbonate header plate for fluid fed to and retrieved from theadsorbent channel; (b) adhesive film; (c) ˜0.011-in. thick serpentineheat exchange fluid channel laser cut in polyimide film; (d) adhesivefilm; (e) a thin (e.g., 0.030-in. thick) heat exchange surface filmfabricated from polyimide or conductive polyimide; a pattern of smallholes was laser machined in the corners of the shim serving the purposeof the metal screens described above [layer (a) of the all-stainlesssteel device]; (f) adhesive film; (g) a polyimide serpentine adsorbentshim, typically 0.050-in. (1.3 mm) thick, machined in the same patternused in the all-metal device (see FIG. 2) and filled with zeolite; (h)adhesive film; (i) a heat exchange surface (e) except without the lasermachined adsorbent screen; (j) adhesive film; (k) serpentine heatexchange channel (e); (l) adhesive film; and (m) 0.25-in thickpolycarbonate header plate for fluid fed to and retrieved from the heatexchange fluid channels.

Metal-Plastic Test Device

Metal-plastic composite devices were also fabricated and tested. A unitwas prepared as described for the all-plastic device except the heatexchange surface films (e) and (i) were replaced with thin copper shims.Results for this type of composite device are shown in FIG. 8.

Another variation of a metal-plastic mesochannel adsorber has beenfabricated and used in a limited number of experiments. It is identicalto the all-stainless steel device described above excepting the centraladsorbent shim (c) is replaced with a polyimide equivalent like thatused in the other metal-plastic and all-plastic adsorbers (e.g.,0.050-in., 1.3 mm thick).

In the all-plastic and the metal-plastic adsorbers, the design of theheat exchangers was the same. In the all-metal adsorber, the design ofthe heat exchangers was somewhat different, but the fluid channelthickness (0.010 in, 0.25 mm) was comparable. The adsorption channel ineach device was packed with zeolite 13× adsorbent (PQ Corp., 180 to 212μm sieve fraction). A test stand stand was assembled to control feed gas(pure CO₂) and heat exchange fluid (water) flow rates and to allowmonitoring adsorber and heat exchange fluid temperatures, pressuredrops, and evolved gas volumes. Type K surface mount and immersion probethermocouples were deployed in all tests; in several tests, a type Thypodermic thermocouple (Omega™) was embedded in the adsorption media tomeasure the adsorption media temperature directly. Temperatures wereoutput and recorded to an Omega data acquisition system on a personalcomputer. The series of valves needed to switch between adsorption anddesorption cycles (FIG. 1) were controlled manually. During desorption,gas was evolved at essentially ambient pressure through a tube to thehead space of an inverted graduated cylinder that was partially filledwith water and whose opening was submerged in a room temperature waterreservoir. The water displaced from the cylinder measured the volume ofevolved gas. To monitor gas evolution as a function of time, the waterdisplacement from the cylinder was video taped for subsequentevaluation. Ideal gas law assumptions were applied to determine theequivalent mass of CO₂ released for comparison to the theoreticalworking capacity.

Example 5

Calculated Productivity

The use of a mesochannel sorption pump, as described herein, provides ameans of process intensification for gas processing by thermal swingadsorption. The productivity, defined as the mass of target gasprocessed per unit volume sorption pump, is a measure of processintensification. The productivity is related to many factors includingcycle rate, gas stream composition, adsorption and desorptiontemperatures and pressures, and adsorbent type and condition.

The Productivity Graph (see FIG. 10 ) shows the productivity of severalsorption devices of the current invention using a set of benchmarkconditions. These include: (a) adsorbent channels filled with clean, dryzeolite 13× particulate to a density of ˜0.67-g zeolite/mL channel; (b)CO₂ adsorbed to equilibrium at 760 mm Hg and 5° C. by flowing a 5° C.heat transfer fluid through the heat exchange channels and pure CO₂through the adsorbent per FIG. 1 a; and (c) desorption of CO₂ at apressure of 760 mm Hg resulting from the flow of 90° C. heat transferfluid through the heat exchange channels (limiting the desorptiontemperature) per FIG. 1 b. A further constraint on this test, is thatthe productivity is defined for a single desorption of the deviceoccurring in 1 minute (or less) from the time the high temperature heatexchange fluid starts flowing through the sorption unit or heating isinitiated. Therefore, the productivity results in the Productivity Graphrepresent the mass of CO₂ desorbed in a single desorption per unitvolume of sorption pump structure subject to the constraints givenabove. The theoretical maximum mass of CO₂ desorbed under theseconditions can be estimated using Equation (2) and the volume anddensity (or mass) of adsorbent contained in the device. As notedpreviously, the actual working CO₂ production of such operations may beless than 100% owing to various factors such as partial loading of theadsorbent with water vapor. Maximum productivity would also not beattained if the sorbent did not reach the temperature of the heatexchange fluid. Accounting for this type of inefficiency, we haveapplied an efficiency factor of 0.85 in our calculations, a factor weexpect to meet or exceed in routine operations. The results in theProductivity Graph are 85% of the maximum theoretical productivity.

The Productivity Graph demonstrates that productivity is a strongfunction of sorption pump design. This is a direct result of thevariation in the amount of adsorbent contained per unit volume ofsorption pump structure. The structure volume is defined by the outerwalls (e.g., plates) of the sorption unit, and it contains the adsorbentmesochannels, the heat exchange channels, and internal header and footerchannels needed to deliver and collect fluids from the heat exchange andadsorption channels. In the cases presented in the Productivity Graph,the sorption pump consisted of 10 adsorbent mesochannels interspersedwith 11 heat exchange channels such that each adsorbent channel wascontacted by two heat exchange surfaces. The height of the heat exchangechannels, the outer wall thickness, the header and footer channel crosssection, and the width of the adsorbent channels (5 cm) were fixed,while the channel lengths and adsorbent mesochannel height (thickness)were varied. The Productivity Graph shows calculated results for 1-, 3-,and 5-cm long channels. At any given channel length, the expectedmaximum productivity increases as the adsorbent mesochannel thickness(height) increases, because the fraction of the device structureoccupied by sorbent increases accordingly. In the limit of infiniteadsorbent channel height, the sorption pump structure volume isdominated by adsorbent, and an asymptotic limit on productivity isreached. In practice, however, the adsorbent channel thickness must belimited to affect rapid heat transfer and rapid cycling for increasedproduction rates (see Examples 1 and 2). The Productivity Graph alsoshows that at a given adsorbent mesochannel thickness, the productivityincreases with increasing channel length. Again, this is due to theincrease in the fraction of structure volume occupied by sorbent withincreasing length. In practice, channel length may be limited because ofpressure drop considerations.

Under the benchmark test conditions specified above, it can be seen thatvarious configurations of our invention will meet or exceed aproductivity of 0.015-g CO₂ per mL-sorption pump structure volume.

Table 1 summarizes estimates of CO₂ productivity that we calculated forsorption compressors described in Karperos, “Operating Characteristicsof a Hydrogen Sorption Refrigerator—Part I. Experimental Design andResults,” Proceedings of the Fourth International Cryogenic Conference,Easton, Md. (1986). The calculations are predicated on the assumptionsof sorbent type, adsorption and desorption temperature and pressureoperating limits, and desorption cycle time used to determineproductivity for the current invention and as described in conjunctionwith the Productivity Graph. For the estimates of the devices inKarperos, however, the operation was assumed 100% efficient, therebyresulting in an estimate of maximum potential productivity. Karperosdescribes the use of a 20% density copper foam within the sorbentchannel to promote heat transfer; in the calculations made here, it wasassumed that 20% of the sorbent channel was occupied by the foam,effectively decreasing the sorbent volume within the compressor.

TABLE 1 Basis of Productivity Calculations for Sorption CompressorDescribed in Karperos, ″Operating Characteristics Of A Hydrogen SorptionRefrigerator,″ Proceedings of the Fourth Int'l Cryogenic Conference(1986) Device 1, FIG. 4 Left Device 1, FIG. 4 Right (as est. from text,Sec (as est. from text, Sec 3 and drawing FIG. 4) 3 and drawing FIG. 4)Compressor Radius (cm) 2.822 2.782 Compressor Height (cm) 51.1 26.64Compressor Volume 1278.5 647.7 (mL) Sorbent Annulus, 2.382 2.382 Rout(cm) Sorbent Annulus, 2.064 2.064 Rin (cm) Sorbent Annulus 50.8 25.4 Ht(cm) Sorbent Volume in 180.5 90.2 Annulus (mL) Sorbent Upper Disk 0 0.22Thickness (cm) Sorbent Volume in 0 3.92 Upper Disk (mL) Sorbent VolumeTotal 180.5 94.2 (mL) Zeolite 13× Mass (g) 120.9 63.1 Maximum Mass CO₂11.6 6.03 desorbed per cycle (g) Maximum Productivity 0.00905 0.00932 (gCO₂/mL compressor)

Example 6

Heat Transfer Power Density

The productivity of a mesochannel sorption pump is in part dependentupon the heat transfer power density that can be obtained in the thermalinteraction between adsorbent channels and heat exchange channels. Acalculation was performed to estimate the heat transfer power densityrequired for a collection of mesochannel sorption pump cells, for thethermochemical compression of CO₂ from 760 mm Hg to a higher pressures,ranging from 0.5 bat to 10 bar. As previously described, the heattransfer power density is the rate at which heat is added to or removedfrom an adsorption cell, in units of watts per cubic centimeter. For thecalculation, a “flow-by” design was assumed, incorporating adsorbentmesochannels containing 13× zeolite with height, width and length,respectively, of 750 μm, 1 cm, and 5 cm, and microchannel heatexchangers with height, width and length, respectively, of 250 μm, 1 cm,and 5 cm. A stainless steel structure was assumed, as was therecuperative heat transfer cycle of Swyulka, where thermal energy fromcells that are cooling are transferred to cells that are heating. The“delta T” (which represents the difference in temperature between thetemperature of desorption and the temperature of adsorption) of eachcycle was varied, with individual calculations assuming a delta T of100° C. or 200° C. Two and four minute cycles were also assumed for thisset of bounding calculations.

The calculations considered the full heating and cooling requirementsfor each cell, including consideration of the thermal mass of the unitsplus the heats of adsorption and desorption. In general, the heattransfer power densities that were calculated from this exercise rangedfrom 1.10 watts per cubic centimeter to 5.99 watts per cubic centimeter,with the highest heat transfer power densities corresponding to theshortest cycle periods, and greatests delta T's per cycle.

The heat transfer power densities that were calculated in this exerciseare of magnitudes that can be obtained in systems that incorporatemesochannel heat exchangers; indeed, it is not difficult to obtain heattransfer power densities that are approximately one order of magnitudehigher, yet with low pressure drops for the heat transfer fluids,suggesting that shorter cycle times would also be achievable formesochannel sorption pumps.

It is also clear from this calculation that, if longer cycle times hadbeen assumed, for example, at about 10 minutes per cycle, the heattransfer power densities would be somewhat less. Mesochannel sorptionpumps therefore are estimated to be able to obtain specificproductivities (output per unit hardware volume) that require heattransfer power densities exceeding 1 watt per cubic centimeter, andperhaps exceeding tens of watts per cubic centimeter.

Example 7

Calculations of Mass Transport Times in Flow-By Mesochannel Adsorbers

Previously, we defined the characteristic mass transport time (t_(mt))in an adsorbent channel as $\begin{matrix}{t_{mt} = \frac{L^{2}}{D_{e}}} & (7)\end{matrix}$It is related to the time required for a substantial degree of massdiffusion to occur at a distance L within the adsorbent. D_(e) is theeffective mass diffusivity for diffusion in a porous medium given by$\begin{matrix}{{D_{e} = {D\quad\frac{ɛ}{\tau}}},} & (8)\end{matrix}$where D is the mass diffusivity of the species in the fluid, and ε and τare the porosity and tortuosity factor, respectively, of the porousmedium.

Another method to assess transient mass transport and the approach tothe equilibrium concentration of gas species within an adsorbent channelis to apply the solution for the problem of unsteady state masstransport in a semi-infinite medium. The problem and solution areroutinely described in the mass and heat transport literature (e.g.,Hines, A. L. and R. N. Maddox, Mass Transfer, Fundamentals andApplications, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1985.).Physically and as applied here, the semi-infinite approximation assumesdiffusion from a surface or reservoir, like the fluid channel in acontactor-based “flow-by” adsorber, at constant concentration C_(Ao) ofspecies A to a medium of vast extent (semi-infinite) initially at somelower concentration C_(A∞). In this case, we treat the adsorbent channelin the “flow-by” system as semi-infinite and assign a value of zero toC_(A∞). The variation in species A concentration C_(A) as a function ofthe semi-infinite diffusion time t_(si) and the distance L within theadsorbent channel can then be estimated from the Gauss error function(erƒ) solution to the semi-infinite diffusion problem: $\begin{matrix}{{1 - \frac{C_{A}}{C_{Ao}}} = {{{erf}\left( \frac{L}{2\sqrt{D_{e}t}} \right)} = {{erf}\left( \eta_{si} \right)}}} & (9)\end{matrix}$The results of erƒ(η_(si)) for η_(si) values ranging from 0 to 3 aretabulated in many mathematical handbooks and in some mass and heattransport texts.

Strictly speaking, the solutions given by Equation (9) apply when theconcentration of species at some point in the medium where diffusion istaking place remains at the initial concentration (C_(∞)). This occurswhen the time is very short and little diffusion has occurred or atrelatively large distances from the diffusion source. It is instructiveto investigate solutions of Equation (9) to diffusion in mesochanneladsorbers, even though the physical situation may not adhere strictly tothe aforementioned conditions, to better understand the relationship ofspecies concentration with time. In fact, the solutions given byEquation (9) do not take into consideration other factors in anadsorption process, such as depletion of gas species as they areadsorbed onto the adsorbent, or some of the boundary constraints (e.g.,walls) that will be present in a practical, mesochannel sorption unit.

Acknowledging the oversimplification of the analysis, we presentsolutions to the semi-infinite diffusion problem for the case ofa-contactor-based, “flow-by” adsorbent microchannel for diffusion of CO₂in N₂ at 298 K and 1 atm (D=1.67×10−5 m²/s),where L=800 μm, into andwithin an adsorbent media with a porosity of 0.5 and a tortuosity factorof 3. The time variation in relative CO₂ concentration (C_(A)/C_(Ao))for these conditions is shown in the Mass Diffusion Graphs (FIG. 9). Inthe upper Mass Diffusion Graph, time is presented in seconds; in thelower graph, the same results are represented in terms of thecharacteristic mass transport time (t_(mt)) for a microchannel of height(h) equal to L.

The Mass Diffusion Graphs confirm that a substantial amount of diffusionhas occurred at t=t_(mt). More specifically, when the semi-infinitediffusion time is equal to the characteristic mass transport time(t=t_(mt)) for a microchannel of 800 μm height, the relative CO₂concentration at 800 μm is 0.48, or 48% of the steady state value. Thiscorresponds to a η_(si) value of 0.5 in Equation (9) and a diffusiontime of 0.23 s.

In general, the Mass Diffusion Graphs indicate that the CO₂concentration at the specified distance increases rapidly with time forshort times up to moderate CO₂ concentrations (e.g., C_(A)/C_(Ao)<0.8),but the rate of change drops off significantly when the relative CO₂concentration is high. For example, 29.2 s are needed to reach arelative CO₂ concentration of 0.95, corresponding to 127-times thecharacteristic mass transport time (t_(mt)). Note that the bulk averageconcentration in the adsorbent channel between the source and a distanceL into the adsorbent channel would be higher than relative concentrationdetermined at L, because the relative concentration at a given time forany distance less than L exceeds the concentration at L. In other words,a concentration gradient exists. Again, the analysis does not evaluategas species depletion due to uptake by an adsorbent, and does not takeinto account some of the boundary conditions that would be present in amesochannel, but it is useful in characterizing and comparing transientmass transport response for various mesochannel adsorber configurations.

As previously described, Viswanathan et al. applied the solution forunsteady state mass transport in a semi-infinite diffusion medium totheir contactor-based “flow-by” adsorber, and reported an estimate of 30seconds for 95% of the CO₂ to reach the zeolite adsorbent (Viswanathan,Wegeng, and Drost, “Microscale Adsorption for Energy and ChemicalSystems”, appearing on the Pacific Northwest National Laboratory website in May 2000). This calculation confirms the potential for rapidmass diffusion within an adsorption microchannel, therefore indicatingthat rapid cycling might be achieved.

Although the devices described in the Examples section were all singlechannel devices; the designs are suitable for multichannel units havingat least comparable working capacity performance on a per hardwarevolume basis.

CLOSURE

While preferred embodiments of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to include all such changes and modifications as fall withinthe true spirit and scope of the invention.

1. A sorption pump comprising: at least 2 adsorption layers; whereineach of the at least 2 adsorption layers comprises an adsorptionmesochannel containing adsorption media; and at least 3 heat exchangerlayers wherein each of the at least 3 heat exchange layers is in thermalcontact with an adsorption layer; wherein each of the at least 3 heatexchange layers comprises at least one microchannel; wherein theadsorption layers are interleaved with the heat exchange layers; andwherein each of the at least 2 adsorption layers has a gas inlet suchthat gas directly contacts the adsorption media without first passingthrough a contactor.
 2. The sorption pump of claim 1 wherein each of theat least 2 adsorption layers comprises a plastic and wherein each of theat least 3 heat exchange layers comprises a metal.
 3. The sorption pumpof claim 1 further comprising, in each of the at least 2 adsorptionlayers, a gas outlet separate from the inlet; wherein the outlet isdisposed such that a gas stream can flow through the inlet, through theadsorption media and out the outlet.
 4. The sorption pump of claim 3wherein the pump possesses capability such that, if the adsorption mediais replaced with an equal volume of 13× zeolite, with a bulk density of0.67 grams per cubic centimeter, and then saturated with carbon dioxideat 760 mm Hg and 5° C. and then heated to no more than 90° C. at 760 mmHg, then at least 0.015 g CO₂ per mL of apparatus is desorbed within 1minute of the onset of heating.
 5. Gas adsorption and desorptionapparatus comprising: at least one adsorption layer comprising anadsorption mesochannel containing adsorption media; and at least oneheat exchanger in thermal contact with the adsorption layer; wherein theadsorption mesochannel has dimensions of length, width and height;wherein the height is at least 1.2 mm; and wherein the apparatuspossesses capability such that, if the adsorption media is replaced withan equal volume of 13× zeolite, with a bulk density of 0.67 grams percubic centimeter, and then saturated with carbon dioxide at 760 mm Hgand 5° C. and then heated to no more than 90 ° C., at 760 mm Hg, then atleast 0.015 g CO₂ per mL of apparatus is desorbed within 1 minute of theonset of heating.
 6. The apparatus of claim 5 comprising: at least 2adsorption mesochannels, each containing adsorption media, interleavedwith at least 3 heat exchanger layers, each heat exchanger layercomprising at least one microchannel.
 7. Gas adsorption and desorptionapparatus comprising: at least 4 adsorption/desorption cells each cellcomprising at least one adsorption mesochannel in thermal contact withat least one heat exchanger; wherein the at least one heat exchangercomprises a microchannel heat exchanger; wherein the adsorption channelcontains adsorption media; the apparatus connected to a heat source anda heat sink; and conduits between each heat exchanger and the heatsource and the heat sink and also conduits between at least one heatexchanger in each cell and at least one heat exchanger in another cell.8. A sorption pump, comprising: an adsorption layer comprising anadsorption channel containing adsorption media; and a mesochannel heatexchanger in thermal contact with the adsorption layer; wherein themesochannel heat exchanger has a fluid flowing therethrough that has ahigh thermal diffusivity, such that the characteristic heat transporttime of the fluid in combination with the mesochannel heat exchanger isa value no greater than 10 seconds; and wherein said fluid is a liquidmetal or a silicone-based fluid.
 9. A multi-cell sorption pump,comprising: at least six sorption cells; wherein each sorption cellcomprises at least one adsorption layer, and at least one heat exchangerlayer; thermal connections connecting each sorption cell to at least twoother sorption cells and to a heat source and to a heat sink, such thateach sorption cell can cycle thermally from adsorption to desorption andback to adsorption by sequentially receiving heat from said at least twoother sorption cells prior to receiving heat from the heat source, andthen sequentially giving up heat to at least two other sorption cellsprior to giving up heat to the heat sink, such that thermal recuperationis provided.
 10. The multi-cell sorption pump of claim 9, wherein theheat source is selected from the group consisting of an electricalresistor, a process technology, solar power, nuclear power.
 11. Themulti-cell sorption pump of claim 9, where the thermal connections areheat switches.
 12. The multi-cell sorption pump of claim 9, wherein thethermal connections comprise fluid loops.
 13. The multi-cell sorptionpump of claim 9, wherein the sorption pump incorporates mesochannelsorption channels, and wherein the sorption pump incorporatesmesochannel heat exchange channels.
 14. The sorption pump of claim 4where the adsorption media is heated to 90° C. by flowing warm water at90° C. through the heat exchange channels.
 15. An air treatment systemcomprising the sorption pump of claim 1, comprising: an oxygen source; afirst sorption cell comprising the sorption pump of claim 1 wherein theadsorption media comprises a water adsorbent; a second sorption cellcomprising the sorption pump of claim 1 wherein the adsorption mediacomprises a water adsorbent; a third sorption cell comprising thesorption pump of claim 1 wherein the adsorption media comprises a CO₂adsorbent; and a fourth sorption cell comprising the sorption pump ofclaim 1 wherein the adsorption media comprises a CO₂ adsorbent.
 16. Asorption pump, comprising: an adsorption layer comprising an adsorptionchannel containing adsorption media; and a mesochannel heat exchanger inthermal contact with the adsorption layer; wherein the mesochannel heatexchanger has a fluid flowing therethrough that has a high thermaldiffusivity, such that the characteristic heat transport time of thefluid in combination with the mesochannel heat exchanger is a value nogreater than 10 seconds; wherein adsorption media fills at least 60% ofthe cross section of at least one portion of the adsorption channel. 17.The sorption pump of claim 16 wherein the adsorption channel has aheight of 1.2 mm to 1 cm.
 18. The sorption pump of claim 16 wherein theadsorption channel has a height of 1 cm or less, and wherein adsorptionmedia fills at least 90% of the cross section of at least one portion ofthe adsorption channel.
 19. The sorption pump of claim 1 wherein theadsorption mesochannels in each of the at least two adsorption layersare at least 50% filled with adsorption media.
 20. The sorption pump ofclaim 1 wherein the each of the at least two adsorption layers comprisea plastic structural material, and wherein each of the at least 3 heatexchanger layers comprise a metal structural material.
 21. The sorptionpump of claim 1 wherein the each of the at least two adsorption layerscomprise adsorption media and interspersed thermally conductivematerials.
 22. The sorption pump of claim 1 wherein the each of the atleast two adsorption layers comprise an adsorption mesochannel that iscut completely through the adsorption layer.
 23. The sorption pump ofclaim 1 wherein the each of the at least two adsorption layers comprisean adsorption mesochannel comprising an adsorption media in the form ofparticles, pellets, foams, felts, or honeycombs.
 24. The sorption pumpof claim 6 wherein the each of the at least two adsorption layerscomprise a plastic structural material, and wherein each of the at least3 heat exchanger layers comprise a metal structural material.
 25. Thesorption pump of claim 5 wherein the adsorption media is in the form ofparticles, pellets, foams, felts, or honeycombs.
 26. The sorption pumpof claim 5 wherein the adsorption mesochannel is at least 50% filledwith adsorption media.
 27. The sorption pump of claim 5 whereinadsorption media fills at least 60% of the cross section of at least oneportion of the adsorption channel.